1. Field of the Invention
The present invention is directed to an X-ray diagnostics installation of the type having an X-ray tube, an X-ray generator, a planar solid state X-ray image converter for generating X-ray images that is composed of a number of individual detectors abutting one another, an image system and a playback device.
2. Description of the Prior Art
Solid state detectors utilized in X-ray imaging are based on active readout matrices of, for example, amorphous silicon (a-Si). The image information is converted in an X-ray converter (for example, cesium iodide, CsI), is stored as electrical charge in the photodiodes of the matrix, and is subsequently read out via an active switch element with a dedicated electronics and is analog-to-digitally converted.
Solid state detectors (FD) utilized in X-ray imaging are based on active readout matrices of, for example, amorphous silicon (a-Si). The image information is converted in an X-ray converter (for example, cesium iodide, CsI), is stored as electrical charge in the photodiodes of the matrix, and is subsequently read out via an active switch element with a dedicated electronics and is analog-to-digitally converted.
FIG. 1 shows such an X-ray diagnostics installation disclosed by German OS 195 27 148 having an X-ray tube 2 supplied with high-voltage and filament voltage by a voltage generator 1. The X-ray tube 2 emits a conical X-ray beam 3 that penetrates a patient 4 and generates a radiation image on a solid state detector that is sensitive to X-radiation 3. The output signal of the solid-state detector, the image data 6, is supplied to an image system 7. The image system 7 can include converters, image memories and processing circuits. The image system 7 is connected to a monitor 8 for the playback of the acquired X-ray images. Operating elements 9 are connected to the other components of the X-ray diagnostics installation via a system control and communication 10.
FIG. 2 shows a cross-section of the solid state detector 5. The basic components of the solid state detector 5 are a solid state pixel matrix, line drivers and amplifiers. The solid state pixel matrix is in turn composed of a layer with a scintillator 11, for example of cesium iodide (CsI), that supplies in the visible photons spectrum to a pixel matrix 12 of amorphous silicon when irradiated with the X-ray beam 3, photons forming a visible X-ray image. As shown enlarged in FIG. 2, each of the pixels or picture elements of this pixel matrix 12 is composed of a photodiode 13 and a switch 14 that is connected to row lines 15 and column lines 16. The pixel matrix 12 is applied on a glass substrate 19.
All pixels of a line are addressed and read out simultaneously by the line drivers. The signals are processed in parallel in a number of amplifiers 18. In the simplest case, an image is progressively read out line-by-line.
Large-area detectors, for example for general radiography, are generally manufactured of a number of plates, i.e. individual detectors with glass substrates and an a-Si layer in order to maximize the yield and avoid the considerable costs for larger systems. Detectors composed of two or four individual plates are standard. Since two plates always differ in detail in terms of their electrical properties and are also driven by different electronics, non-linearities that make the various plates visible in corrected images can arise under certain conditions. Due to the large xe2x80x9ccorrelatedxe2x80x9d structures the minutest differences in gray scale value are already visible beginning with a digital unit (du). Further, the differences in gray scale value are generally not described by a single offset per plate but are dependent on the local dose and on the physical characteristics of the plates themselves, so that continuous changes in gray scale value difference (gray scale levels) can occur at a plate-to-plate transition. These differences in gray scale value generally occur non-deterministically.
German OS 199 15 851 discloses a method for processing the pixel image signals supplied by a solid state image sensor composed of a pixel matrix with a dark reference zone, whereby an offset correction value is formed from the dark reference zone.
German OS 196 40 999 discloses a method for covering errors in images encoded by blocks, whereby a correction at the block edges is estimated by a straight line or a polynomial.
An object of the present invention is to provide an X-ray diagnostic installation of the type initially described wherein a uniform transition arises at the abutting locations of the plates and the butting structurexe2x80x94the abutting locations of the platesxe2x80x94is no longer visible.
The object is inventively achieved in an image system having a detection unit that locally recognizes the amplitude of continuously changing gray scale levels at the abutting locations and generates a correction signal corresponding to the amplitude of the gray scale levels, and a correction unit for the continuous adaptation of the gray scale levels at the abutting locations. As a result, continuous changes in gray scale that occur between two plates, i.e. along an abutting location, can be corrected such that the abutting location of the plates is no longer optically visiblexe2x80x94i.e. can no longer be perceived. To that end, the abutting locations between the respective plates are made uniform of smoothed steady in the gray scale transition. Without this method, the plates are generally visible due to different gray scales, which is an important disadvantage compared to a manufacturer who can manufacture the detectors from a single plate. The special characteristic of the method is that the gray scale matching at the abutting location is locally undertaken, and thus different gray scale levels at different locations can be taken into consideration. The apparatus analyzes the current image and is suitable for non-deterministic gray scale value transitions.
Inventively, the detection unit can include a subtraction stage, an averaging stage and/or a filter stage.
It has proven advantageous when the correction unit includes a multiplication stage for multiplying the correction factor of the detection unit by a location-dependent factor within a region. The multiplication unit can thereby multiply the correction factor by an x-dependent or y-dependent function that decreases steadily proceeding from the abutting location.
The detection unit can include an addition stage that is connected to the multiplication stage.
The correction can be manually influenced by allowing the size of the region, i.e. the range of the correction, to be adjustable.
Inventively, the solid state X-ray image converter can be composed of two individual detectors having only one abutting edge, or of four individual detectors having two abutting edges that form a cross of abutting edges. Other arrangements are also conceivable, for example one having 3*3 individual detectors as well.